In order to understand the full range of the present invention, it is necessary to discuss some of the efforts of the past to assist in the preparation of food and, in particular with regard to cookware, to make cooking and clean-up less time consuming. In recent years many different types of cookware; such as frying pans, cookie sheets and the like, have been developed which have non-stick surfaces to prevent substances from sticking to the cookware surface during food preparation and thus aid in clean-up. There also have been aerosol sprays which are sprayed into pan surfaces which prevent sticking, thereby aiding in clean-up. One of the primary advances in this art was the development of polytetrahaloethylene, such as polychlorotrifluoroethylene, and in particular, polytetrafluorethylene which is marketed under the name Teflon (a registered trademark of the E. I. duPont de Nemours Co. of Wilmington, Del. 19898). The same polymer is manufactured by other companies and may be marketed under other trademarks.
Polytetrafluoroethylene is noted for its anti-stick qualities, relative inertness, high temperature resistance and in many forms is approved by the Federal Food and Drug Administration (FDA) for food contact use. The Teflon coated frying pans, metal cookie sheets, etc. have a layer of polytetrafluoroethylene adhered directly to the metal surface which provides the non-stick characteristic to permit easy clean-up. Although improvements have been made over the years in polytetrafluoroethylene coated utensils, there are still attendant problems. For example, the coating soon becomes scored or scratched through the use of sharp utensils or improper use of scouring pads during cleaning. Additionally, the fully adherent polytetrafluoroethylene layer is prone to trap odors from rancid or decomposed shortening and food, thus imparting an undesirable taste to ensuing cooked or baked foods. Washing out the malodorus or bad tasting components permeated through the polytetrafluoroethylene coating is difficult to achieve because only one surface of the polymer is exposed. What is needed is a totally removable non-adherent interlayer or liner between the utensil and the baked or cooked items so that the liner can be divested of any permeated or residual food components by soaking or washing in normal detergents on both sides.
Polytetrafluoroethylene has not only been used in cookware and numerous other coating applications but it has been produced as a thin film for additional applications such as cookware liner, electrical tape, and other industrial applications. The film can be produced by various methods including skiving and casting which are generally described in the background description of U.S. Pat. No. 2,852,811. Although thin polytetrafluoroethylene film has been used as a cooking liner in the past, it did not meet with great acceptance. The general physical characteristics of such films and particularly the transparency led the consumer to confuse the polytetrafluoroethylene film with ordinary polyethylene films such as are used in sandwich bags or as coverings for freshly cleaned clothing. As is commonly known, polyethylene should not be used in conventional ovens, such an error would result in the material melting or burning with subsequent harmful vapors being released.
A more recent advance in food preparation or cooking is the advent of the microwave energy ovens which are now commonplace. One of the primary problems with the microwave energy is that it fails to provide the proper so-called browning and/or crisping to the foods which are normally expected to have such a quality. Therefore, foods of this type after being heated or cooked in the microwave, do not possess the degree of appeal and taste normally expected.
Many attempts have been made to correct this lack of browning when microwave energy is used. One attempt to solve the browning problem is the inclusion of various modifications such as an electrical broiler element in the microwave oven to produce the short-wave energy needed to brown food. Another solution has been to use various types of eatable coatings, which only add the appearance of surface browning.
Other attempts at solving the browning problem have been directed at modifications of the microwave energy itself by using special utensils, pans and packages to convert the high frequency microwave energy by resistance losses into heat. Heat gives the requisite browning effect and coloration to the food.
Some of these special utensils, pans and packages are illustrated in U.S. Pat. Nos. 3,302,632; 3,701,872; 3,777,099; 4,190,757; 4,230,924 and 4,268,738. Of particular interest with respect to the present invention are the special packages illustrated in the last three above-mentioned patents.
U.S. Pat. No. 4,190,757 to Turpin discloses a disposable microwave shipping, heating and serving package composed of a paperboard carton and a lossy microwave energy absorber which becomes hot when exposed to microwave radiation. The absorber is in a conductive heat-transfer relationship with the food product and the package and is usually bonded to a structural supporting sheet such as aluminum foil. The metal foil sheet is adapted to reduce by a specific amount the direct transmission of microwave energy into the food by providing holes of selected size within the sheet. Besides being expensive to manufacture and not reuseable, these types of packages do not provide intimate contact with all parts of the food being prepared and must be customized for each food item so that the package must be supplied by the food processor.
U.S. Pat. No. 4,230,924 to Brastad et al discloses a flexible dielectric substrate in the form of a sheet of plastic or paper having a metallic coating thereon. The metallic coating is sub-divided into a number of metallic islands or pads with non-metallic gaps or strips between. The size of the gaps can be varied and, thus, permit selected amounts of microwave energy to be transmitted through the sheet. Although the sheet is described as flexible, the foil sheet detracts from its flexibility and prevents it from being draped or wrapped around the food to produce an intimate contact.
U.S. Pat. No. 4,268,738 to Flautt, Jr. et al discloses a material which can be used as a package bag or wrapper which has a microwave moderator that enables microwave cooking of frozen foods at relatively high microwave oven power levels without requiring pre-cooking, defrosting or overpower level changes. One species of the moderator is a wrapper which comprises a perforated sheet of microwave reflective material; for example, aluminum foil. The perforated sheet is provided with a plurality of generally uniformed spaced apertures which are sufficiently large with respect to the wave length of the microwave energy that a substantial portion of the energy directed at the moderator will pass through. The perforated sheet can be sandwiched between thermoplastic films which are substantially transparent to microwave energy and have a relatively low dielectric loss factor and which are substantially impervious to other vapors or liquids encountered in cooking. Examples of such films are polypropylene, polyethylene, fluorocarbons and polyimides. Again, this type of package although useful is expensive to manufacture and is normally not reuseable when made into packages as suggested in the disclosure.